Food Biochemistry and Food Processing

11 Starch Synthesis in the Potato Tuber 259

starch degradation in leaves of the model species
Arabidopsis thaliana(Fig. 11.5). The phosphoryla-
tion of starch granules by glucan water dikinase
(GWD, R1) has been found to be essential for the ini-
tiation of starch degradation in leaves and tubers
(Ritte et al. 2002, Ritte et al. 2004). Transgenic potato
plants (Lorberth et al. 1998) andArabidopsismutants
(Yu et al. 2001) with decreased GWD activity showed
a decrease in starch-bound phosphate and were
severely restricted in their ability to degrade starch,
leading to increased starch accumulation in leaves.
The underlying mechanisms are still unknown. Phos-

phorylation of starch may change the structure of the

granule surface to make it more susceptible to enzy-

matic attack or may regulate the extent to which de-

gradative enzymes can attack the granule.

It is generally assumed that the initial attack on

the starch granule is catalyzed by -amylase (endo-

amylase). This enzyme catalyzes the internal cleav-

age of glucan chains from amylose or amylopectin,

yielding branched and unbranched -1,4-glucans,

which are then subject to further digestion (Fig.

11.5). Debranching enzymes (isoamylase and pullu-

lanase) are needed to convert branched glucans into

Figure 11.5.Outline of the pathway of plastidial starch degradation. The scheme is mainly based on recent molecu-
lar studies in Arabidopsisleaves (Ritte et al. 2002, Smith et al. 2003, Chia et al. 2004). After phosphorylation of glu-
cans on the surface of the starch granule via glucan water dikinase (GWD; R1-protein), starch granules are attacked
most probably by -amylase and the resulting branched glucans subsequently converted to unbranched -1,4-
glucans via debranching enzymes (isoamylase and pullulanase). Linear glucans are metabolized by the concerted
action of -amylase and disproportionating enzyme (D-enzyme, glucan transferase) to maltose and glucose. The
phosphorolytic degradation of linear glucans to Glc-1-P (glucose-1-phosphate) by -1-4-glucan phosphorylase (-1-
4-GP) is also possible, but seems of minor importance under normal conditions. Most of the carbon resulting from
starch degradation leaves the chloroplast via a maltose transporter in the inner membrane. Subsequent cytosolic
metabolism of maltose involves the combined action of glucan transferase (D-enzyme), -1-4-glucan phosphorylase
(-1-4-GP), and hexokinase (HK). Glucose transporter (GTP) and triose-P/Pi-translocator (TPT) are also shown.